Lecture 3 - Machining Process Modelling and Development Flashcards
(37 cards)
Two main strands of development in machinig
application specific - creating a component previously impossible (or very expensive)
process specific - enable tech (tools) to operate further and further beyond parameters (cut harder materials, faster with less lube)
Developments of application specific in HVM
micro milling - very small mechanical parts (biomedical + control devices)
low productivity which is difficult to develop as models/theories of macro milling break down at this scale
Development of process specific in HVM
Minimum Quality Lubrication and Cryogenic machining
conventional lube/coolant is an emulsion of oil in water - not good for environment
Development process to use minimum amount, or use different (specific outlets spray etc)
Drivers of development in machiniing
reduce different operations extend tool life shorter machining time (higher MRR) optimisation of tool change reduction of tool change time optimal tool prepartion
How is productivity increased in machining
optimising operation sequence, cutting tool geom and design, selection cutting parameters
Other cost reduction: using less lube and producing less chips, near net shape technology as starting point, smaller batch size
How has cutting speed evolved
slow with carbon tool steel
high speed steel increased
carbon steel -> cemented carbon -> coated cemented carbide -> ceramics -> cast iron -> cBN and PCDO
Issues with increasing speed
high forces, hotter
How does modern techniques for cutting technology development compare to traditional
Originally make tool test see when it breaks, takes long time
Now FEA, matlab, predict performance check things you wouldnt normally
Models can go beyond current physical limits
What is the main advantage of two part tools
inserts can use more exotic materials
better heat resistance
retain a cutting edge better
easier to heat treat inserts
What is HSS
High speed steel tools, developed pre WW2 cheap easy to shape and resharpen but wear away quickly can melt (TiN Coatings can help) Grey block
Where are HSS tools still used
lower speed machining - gear hobbing and most drilling
What are tungsten carbide (WC) tools
developed as machining became more demanding, more expensive than HSS, but still cheap as inserts
easy to shape (sintered)
good abrasion and heat resistance - maintain cutting edge for longer at faster speeds and higher temps
often coasted in TiN - gold colour
What has HVM demanded beyond WC and HSS tools
go faster, take bigger cuts, increase productivity
near dry or dry machining (no expensive lube)
What are tungsten carbide tools limited by
WC limited by surface speed (heat generation at cutting face)
Feed per tooth (material toughness)
What are the two development for tools from HVM
Ceramics and superhards
What do ceramic composites either have in
aluminium oxide
silicon nitride
matrix with added materials
What added materials can be given to ceramic tools
zirconia added to Aluminium oxide for crack inhibition
SiC whiskers for toughness
TiN for toughness and thermal conductivity
What is the advantage of ceramic inserts
much better heat resistance
retain their mechanical properties (strength at higher temps)
begin to soften at 2200 rather than 870
need non traditional processes to work
What is the disadvantage of ceramic tools
brittle so will fail suddenly
How do ceramic tools behave
silicon nitride inserts perform similarly to carbide tools but at much higher temperatures
alumina zironia tool have improved toughness
alumina-TiC exceptional abrasion resistance
What are the types of superhard tools
Polycrystalline cubic boron nitride (PCBN)
Pollycrystalline Diamond (PCN)
then either straight PCBN (90%), high content (80%) or low content (70%)
How are superhard inserts made
using high temp/pressure sintering
Whats the issue with PCN and PCBN
very very expensive
How is the performance of PCBN tools controlled
grain size and alloy content during manufacture
higher % of CBN = higher fracture toughness
